1. Field of the Invention
The present invention relates to a liquid crystal display device having wide viewing angle characteristics and high performance of image quality capable of maintaining a high contrast ratio.
2. Description of the Related Art
A liquid crystal display has a strength, compared to a CRT (Cathode Ray Tube, generally referred to as CRT), or a conventional mainstream of display devices, in that the display device can be formed thin and light. As development of a technique for improving image quality, use and market for liquid crystal displays have been expanded.
In recent years, for example, a monitor of a desk top personal computer or for printing or designing, use of a liquid crystal television has been increased. With increase of use as a liquid crystal television, there are demanding requests for preferable color reproducibility and a higher contrast ratio. As a luminance in a black state has a finite value (hereinafter referred to as a black luminance) rather than zero, a contrast ratio of a liquid crystal display device is defined by a value obtained by dividing a luminance in a white state (hereinafter referred to as a white luminance) by a black luminance, the white luminance being defined by effective transmissivity of a liquid crystal panel.
As a luminance of a liquid crystal display device depends on a viewing angle, the above-described contrast ratio is often discriminated between a front contrast ratio and a viewing angle contrast ratio. The former is a value defined by a luminance value measured on the front surface of a panel, being specifically a luminance defined by a two-degree viewing field, that is, a luminance measured on a luminance photometer, while the latter is a contrast ratio obtained when viewing a liquid crystal display from a oblique direction and being strongly dependent on viewing angle characteristics of a polarizing plate.
A liquid crystal display mode affects viewing angle characteristics. In general, in a display mode in which a direction of an electric field for driving liquid crystal is perpendicular or nearly perpendicular to a pair of substrates holding a liquid crystal layer in-between, that is, a plane of the liquid crystal display device (a plane viewed by an observer), a contrast ratio (a viewing angle contrast ratio) for a oblique direction significantly drops. In particular, in a black state, a contrast for a oblique direction drops due to remarkable light leakage in a oblique direction. In order to reduce such oblique light leakage, generally, an optical retarder is used or viewing angle characteristics of a polarizing plate are compensated for.
Here, as a liquid crystal display mode, an IPS (In Plane Switching) type liquid crystal display device is available in which a plurality of electrodes are formed on one substrate and the liquid crystal is driven by an electric field formed between the electrodes. In such an IPS type liquid crystal display device, liquid crystal molecules are rotated by applying an electric field in a direction of a plane in parallel to a pair of substrates holding a liquid crystal layer in-between. Although on principle, the liquid crystal layer has wide viewing angle characteristics, the IPS type liquid crystal display device employs a phase compensation technique for further compensating for the viewing angle characteristics of a polarizing plate.
Note that, as a general technique for improving viewing angle characteristics, Japanese Patent Laid-open Publication No. 2005-128498 discloses an IPS type liquid crystal display device using a retardation film, in which the upper polarizing plate and lower polarizing plate in use each has positive uniaxiality.
As a factor which decreases a contrast ratio, not only the above-described viewing angle characteristics but also partial depolarization by respective members in a liquid crystal panel (formed using a polarizing plate, a substrate, and a plurality of kinds of layers in the substrate, except a backlight unit) has significant influence. In particular, a black state in a normally black type liquid crystal display device (black state with no electric field being applied) generally used in a liquid crystal television is displayed by blocking a polarized light passed through a polarizing plate provided on a backlight side (hereinafter referred to as a polarizer) by a polarizing plate provided on an opposite side of a backlight side (hereinafter referred to as an analyzer; in a normally black type liquid crystal display device, an analyzer and a polarizer are positioned such that respective polarization axes are orthogonal to each other at substantially 90 degrees).
A polarized light having passed through the polarizer is subjected to partial depolarization when passing through the liquid crystal panel, partially depolarized light leaks from the analyzer.
This increases a luminance in a black state and thereby deteriorates black state quality. Notes that a partial depolarization member and influence thereof is reported in “M. Yoneya et al., J. Appl. Phys., 98 (2005), p. 016106.” “Y. Utsumi et al., EuroDisplay' 05, P-27.” and “Y. Utsumi et al., IDW'07, LCTp7-12L”. These documents include descriptions about light leakage due to scattering by fine pigment particles in a color filter layer or disordered orientation of liquid crystal and so forth. Further, the document “Y. Utsumi et al., IDW'07, LCTp7-12L.” reports that there is a correlation between intensity of light leakage from a liquid crystal layer and an average elastic constant (an average of a splay elastic constant K1, a twist elastic constant K2, and a bend elastic constant K3), and that increase of the average elastic constant results in decrease of light leakage.